This application claims priority under 35 U.S.C. 119 to Japanese Patent Application No. 11-052948 filed on Mar. 1, 1999, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method and a device for measuring a substance concentration in a liquid. More particularly, the present invention relates to a method and a device for optically measuring a substance concentration in a liquid.
2. Discussion of the Background
A liquid containing one or more substances that are completely dissolved is called a solution. The liquid that dissolves the substances in the solution is called a solvent, and a substance dissolved in the solution is called a solute. A liquid containing one or more substances that are dispersed (i.e. not completely dissolved) is called a dispersing liquid or a dispersion. The liquid in a dispersion is called a dispersion medium or a dispersing liquid, and a substance dispersed in the dispersing liquid is called a dispersoid.
Measurements of concentration of a substance such as a solute or a dispersoid, in a liquid such as a solution or a dispersing liquid, are performed in various fields. As an example, an image forming apparatus such as a laser printer, a photocopier, a facsimile machine, etc., uses a liquid developer which is a solution or a dispersing liquid. The liquid developer includes a toner as a solute and/or a dispersoid, and a carrier liquid as a solvent and/or a dispersion medium. The toner in the liquid developer is electrically charged and therefore adheres to an electrostatic latent image on an image bearer, such as a photoconductor drum. Thus, the electrostatic latent image is developed to a toner image.
In an image forming operation, measuring the toner concentration in the liquid developer and keeping the toner concentration within a certain range according to the measurement is important for forming quality images. Otherwise, a formed image may be degenerated, having, for example, background soiling, a low image density, etc. For measuring the toner concentration in the liquid developer, a sensor having a light-emitting device and a light-receiving device is known.
In a first such sensor, the light-emitting device and the light-receiving device are disposed facing each other and separated by a predetermined distance. The light-emitting device emits light toward the light-receiving device through the liquid developer, and the light-receiving device receives the emitted light. A part of the emitted light is absorbed by the liquid developer. Therefore the intensity of the received light is decreased in comparison with that of the emitted light. As the toner concentration increases, the received light decreases, and as toner concentration decreases, the received light increases. Thus, the toner concentration in the liquid developer is measured based on the intensity of light received by the light-receiving device.
In a second such sensor, both the light-emitting device and the light-receiving device are disposed to face a light reflecting body to which a liquid developer has been applied. The toner concentration is also measured based on the intensity of light received by the light-receiving device. As the light reflecting body, for example, a photoconductive drum, an intermediate transfer belt, a developing belt, etc., may be used.
Generally, a carrier liquid, such as a solvent and/or a dispersion medium, is relatively transparent and a toner is relatively opaque or a light absorbing substance. Therefore the optical transmittance of the liquid developer varies depending upon the toner concentration in the liquid developer. Accordingly, in both types of the above-described sensors, toner concentration may be accurately measured using the output of those light-receiving devices over a certain range. However, as recognized by the present inventors, the measurement of a relatively wide range of toner concentrations causes some difficulties, such as a lack of linearity between the output of the sensor and the toner concentration.
FIG. 1 is a graph illustrating a relationship between a toner concentration in a liquid developer and an output voltage of the second above-noted sensor. For generating the data in the graph, a 50 micrometer thick layer of liquid developer was applied to a light reflecting body opposite the light-emitting device and the light-receiving device. As illustrated, at relatively low and high toner concentrations, the gradients of tangents to the curve become smaller in comparison to the middle range toner concentration. In other words, the sensitivity of the toner concentration sensor becomes smaller at relatively low and relatively high toner concentrations. In addition, at a low toner concentration, the output voltage approaches the saturated output voltage Vm of the sensor, and at a high toner concentration, the output voltage approaches zero volts.
FIG. 2 is a graph illustrating a relationship between the thickness of a liquid developer on the light reflecting body and the output voltage of the second above-noted sensor at different toner concentrations of 10, 15, 10 and 25% in the liquid developer. Referring to FIG. 2, the output voltage of the sensor approximately decreases with increasing thickness of the developer. At a relatively thin developer thicknesses, the output voltage becomes close to a saturated voltage Vm, and at relatively thick developer thicknesses, the output voltage becomes close to zero volts for any concentration of the developer.
As another example, U.S. Pat. No. 5,678,126 describes a toner concentration sensor and a method utilizing a light source, a light splitter, and two light detectors.
The present invention has been made in view of the above-discussed and other problems and to overcome the above-discussed and other problems associated with the background methods and apparatus.
Accordingly, one object of the present invention is to provide a novel method and device for measuring a substance concentration in a liquid in a relatively simple manner and that can measure a relatively wide range of substance concentrations in a liquid.
These and other objects are achieved according to the present invention by providing a novel method and device for measuring a substance concentration in a liquid wherein a continuously variable thickness of the liquid is formed, and light is emitted such that a portion of the light passes through the formed continuously variable thickness of the liquid, an electrical signal is generated according to the portion of the light that has passed through the formed continuously variable thickness of the liquid, and the substance concentration in the liquid is determined based on the generated electrical signal.